1. Field of the Invention
Methods and apparatuses consistent with the present invention relate to an orthogonal frequency division multiplexing (OFDM) transceiving system, and more particularly, to a multi-input multi-output (MIMO)-OFDM transceiving system that uses a plurality of transmission antennas and a plurality of reception antennas.
2. Description of the Related Art
FIG. 1 is a block diagram of a conventional MIMO-OFDM transmitter. Referring to FIG. 1, the conventional MIMO-OFDM transmitter includes error correction coding (ECC) units 11, symbol mappers 12, serial-parallel conversion units 13, pilot insertion units 14, inverse fast Fourier transformation (IFFT)/guard interval (GI) insertion units 15, and DAC/transmission units 16. In particular, in FIG. 1, Nt is equal to the number of transmission antennas (reception antennas), and Nc is equal to the number of subcarriers generated by each of the serial-parallel conversion units 13. A substream mentioned hereinafter denotes a data stream for each of the transmission (reception) antennas.
Each ECC unit 11 performs ECC on data corresponding to a corresponding substream in order to correct an error that may be generated in the data during data transmission. Each symbol mapper 12 maps corresponding data on which ECC has been performed by each ECC unit 11 to data symbols having complex number forms that correspond to modulation signal points.
Each serial-parallel conversion unit 13 allocates the parallel data symbols to Nc subcarriers by converting corresponding data symbols corresponding to the result of the mapping performed by each symbol mapper 12 into parallel data symbols. Each pilot insertion unit 14 receives corresponding subcarriers from the serial-parallel conversion units 13 and inserts pilots for estimating subcarriers' channels into the received subcarriers.
Each IFFT/GI insertion unit 15 generates time-domain transmission signals by performing IFFT on the subcarriers corresponding to the results of the pilot insertions made by each corresponding pilot insertion unit 14, and inserts guard intervals (GIs) for preventing interferences between the data symbols included in the transmission signals into the respective time-domain transmission signals. Each DAC/transmission unit 16 converts the digital transmission signals corresponding to the results of the GI insertions made by each corresponding IFFT/GI insertion unit 15 into analog transmission signals, and simultaneously transmits the analog transmission signals via Nt transmission antennas.
FIG. 2 is a block diagram of a conventional MIMO-OFDM receiver. Referring to FIG. 2, the conventional MIMO-OFDM receiver includes reception/ADC units 21, GI removal/FFT units 22, a channel estimation unit 23, parallel interference cancellers (PICs) 24, MIMO signal detection units 25, symbol demappers 26, error correction decoding units 27, a sub-stream selection unit 28, an ECC unit 29, a symbol mapper 210, and a replication unit 211.
Each reception/ADC unit 21 simultaneously receives corresponding analog transmission signals from the MIMO-OFDM transmitter of FIG. 1 via a plurality of reception antennas, and converts the analog transmission signals into digital reception signals. Each GI removal/FFT unit 22 removes the GIs from the digital reception signals corresponding to the results of the conversions performed by each reception/ADC unit 21. Each GI removal/FFT unit 22 also performs FFT on the GI-free reception signals in order to recover the subcarriers to which the data symbols have been allocated.
The channel estimation unit 23 estimates the channels of subcarriers recovered by all of the GI removal/FFT units 22, using the pilots included in the subcarriers. Each PIC 24 subtracts a replica signal produced by the replication unit 211 from the corresponding subcarriers recovered by each GI removal/FFT unit 22. Such a replica signal is used when substreams having errors exist, and corresponds to subcarriers corresponding to a substream properly recovered by the MIMO-OFDM receiver, the subcarriers having data symbols allocated thereto.
The MIMO signal detection units 25 receive the recovered subcarriers from the GI removal/FFT units 22 and divide the received subcarriers into subcarriers that correspond to each of the substreams, based on information about the channels estimated by the channel estimation unit 23. Each symbol demapper 26 receives a corresponding group of subcarriers from the MIMO signal detection units 25 and maps the data symbols allocated to the received subcarriers into binary data corresponding to each corresponding substream. Each error correction decoding unit 27 receives corresponding binary data from each corresponding symbol demapper 26 and corrects an error of the received binary data by using an error correction code included in the binary data.
The substream selection unit 28 selects a properly recovered substream from among the substreams recovered by the error correction decoding units 27. The ECC unit 29 performs ECC on the substream selected by the substream selection unit 28. The symbol mapper 210 maps the substream on which ECC has been performed by the ECC unit 29 to data symbols having complex number forms. The replication unit 211 produces the replica signal, which corresponds to the subcarriers to which the data symbols output from the symbol mapper 210 have been allocated.
As described above, when substreams having errors exist, the conventional MIMO-OFDM receiver re-performs ECC with respect to the properly recovered substream and performs symbol mapping, thereby producing a replica of a transmission signal corresponding to the properly recovered substream. The conventional MIMO-OFDM receiver also subtracts the replica signal from the reception signals and re-performs a signal division operation with respect to the residual signals. Accordingly, the number of substreams to be equivalently error-correction-decoded is reduced, and the degree of signal division is improved. This transceiving process repeats until all of the substreams are properly recovered.
However, in such a conventional MIMO-OFDM transceiving method, when all the substreams have errors, producing a replica signal for a properly recovered substream is impossible. Thus, in such a conventional MIMO-OFDM transceiving method, parallel interferences between substreams cannot be cancelled. In other words, if a replica signal for a substream having an error is used to cancel the parallel interferences between substreams, the error included in the substream spreads to the other substreams, and thus the other substreams are mapped into wrong data symbols. This hinders generation of a proper replica signal. Therefore, the degree of signal division is greatly degraded.